(a) Field of the Invention
This invention relates to a process for removing dibasic acids, such as maleic acid and citraconic acid, contained methacrylic acid which has been obtained by subjecting a compound having 4 carbon atoms such as isobutylene, tertiary butanol, methacrolein or isobutyl aldehyde to catalytic oxidation with a molecular oxygen containing gas in the presence of steam.
(b) Description of the Prior Art
Methacrylic acid obtained by the catalytic oxidation of a compound having 4 carbon atoms, such as isobutylene, tertiary butanol, methacrolein or isobutyl aldehyde, with molecular oxygen in the presence of steam in a one-step or two-step reaction can be purified to a high-purity product by a usual rectification method such as extraction or distillation. It has however been difficult to achieve complete removal of impurities which are contained in trace amounts.
As these trace impurities, methacrylic acid has been found to contain dibasic acids such as maleic acid and citraconic acid besides protoanemonin, and monobasic acids such as acetic acid, propionic acid, isobutyric acid and acrylic acid byproduced upon synthesis of methacrylic acid. It has also been found that these dibasic acids accompany methacrylic acid due to sublimation, vapor-phase association and/or the like upon distillation and when methacrylic acid is employed in a polymerization reaction, they undergo crosslinking and the like to impair the polymerization properties of methacrylic acid.
It is therefore desirous to remove such dibasic acids completely in the purification step of methacrylic acid. Among these by-products, complete removal of dibasic acids and protoanemonin has been considerably difficult.
As a process for the removal of protoanemonin, Japanese Patent Laid-Open No. 44337/1984 discloses to add a bisulfite to an aqueous solution of methacrylic acid. Regarding a removal process of dibasic acids on the other hand, it is disclosed in Japanese Patent Laid-Open No. 99434/1983 that when an aqueous solution of methacrylic acid is extracted with a solvent subsequent to addition of a basic material thereto, aromatic carboxylic acids, maleic acid, polymers and tar-like matter are either modified or converted into their salts and their extraction is rendered more difficult, thereby making it possible to allow them to remain in the extraction residue and hence to achieve their removal. However, the present inventors are not aware of any publications in which dibasic acids such as maleic acid and citraconic acid are dealt with in detail.
According to the present inventors' experiences, it is necessary to increase the reflux ratio to an extremely high level or to increase the number of stages if one wants to separate and remove trace amounts of dibasic acids from methacrylic acid by distillation. Such an approach is however disadvantageous in both energy consumption and initial investment and moreover, is difficult to remove such impurities completely.
In addition, the present inventors tried to remove maleic acid and citraconic acid by the above-described process, i.e., by adding a basic material to methacrylic acid and then extracting the resultant mixture. Although a relatively high removal rate was achieved with respect to maleic acid, it was unable to lower the content of citraconic acid to any practically acceptable level. It was also confirmed that resultant salts of maleic acid and citraconic acid partly precipitated and dispersed in the extract and deposited in subsequent steps to close up outlet piping.